Refrigeration control system



May 1940- A. B. NEWTON I REFRIGERATION CONTROL SYSTEM Filed March 1'7, 1938 R O T N E V N Alwin) B- Newiorn Y W #M ATTORNEY Patented May 14, 1940 UNITED STATES PATENT OFFICE REFRIGERATION CONTROL SYSTEM Application March 17, 1938, Serial No. 196,451

18 Claims.

This invention relates to control systems for mechanical refrigerating apparatus and more particularly to such a control system embodying an overload cut-out operated upon overloading of the compressor motor.

In conventional refrigerating systems it has been impractical in the past to utilize an automatic resetting overload cut-out. For example, in a refrigerating control system having switching means operated in response to changes in temperature, suction pressure and high pressure and an automatic resetting overload cut-out in series therewith it has been found that (1) the compressor may be caused to cycle on and off on the overload cut-out with the evaporator temperature below freezing whereby frost continually builds up on the evaporator in greater and greater amounts which greatly decreases the cooling effect of the evaporator and hence the efliciency of the refrigerating apparatus, (2) the overload cut-out may be caused to stop the compressor upon the occurrence of high head pressures and if automatic in operation it may try to restart the compressor against the high head pressure conditions with the result that an extremely high starting torque is placed on the motor to again trip the overload cut-out, this operation, which may continue indefinitely, being extremely detrimental to the compressor motor,

and (3) unless the automatic resetting overload cut-out is of the snapvaction type there will be a rapid succession of starting and stopping of the compressor which needless to say is also extremely detrimental to the compressor motor. To eliminate these difficulties manually reset overload cut-outs have been resorted to which must necessarily require personal attention.

It is therefore the prime object of this invention to provide a control system embodying an automatic resetting overload cut-out for a mechanical refrigerating apparatus which obviates the above objections.

In carrying out this invention it is contemplated to utilize a refrigeration control apparatus which may be of the type shown and described in application Serial Number 196,447 filed by Albert L. Judson and Carl G. Kronmiller on March 17, 1938, in combination with an automatic resetting overload cut-out. In this Judson and. Kronmiller apparatus there is provided a starter or relay for controlling the operation of the compressor motor, the starter orrelay having a control circuit including switching means operated in response to variations in suction or low pressure and high or head pressure, the arrangement such that when the relay or starter is dropped out to stop the compressor motor, it cannot again be pulled in to start the compressor motor until the suction or low pressure has increased to a predetermined high value indicative of defrosting and until the high or head pressure has decreased to a predetermined low value which will impose a minimum starting torque on the compressor motor. By locating an automatic resettingoverload cut-out in the control circuit thereof the above outlined difficulties are obviated.

Accordingly, a more specific object of this in vention is to provide a control system for a mechanical refrigerating apparatus embodying an automatic resetting overload cut-out in combination with a control apparatus which prevents starting of the compressor until defrosting has occurred and until the high or head pressure decreases to a minimum whereby (1) upon the occurrence of an overload condition the compressor motor cannot be restarted until defrosting of the evaporator has occurred whereby building up of frost on the evaporator with consequent decrease in efliciency of the refrigerating apparatus is prevented, (2) upon the occurrence of an overload condition the compressor motor cannot be restarted until the high or head pressure decreases to a minimum whereby the starting torque is decreased to a minimum which eliminates one of the main causes of overload shut-downs, and (3) rapid succession of starting and stopping of the compressor motor are entirely eliminated and the use of snap action overload cut-outs is not required.

Another object of this invention is to locate the overload cut-out of this invention in the compressor motor so that the rate of cooling or resetting of the same will be in accordance with the temperature of the compressor motor.

Other objects and advantages will become apparent to those skilled in the art upon reference to the accompanying specification, claims and drawing, in which Figure 1 diagrammatically discloses one form of this invention.

Figure 2 is a diagrammatic illustration of a form of automatic resetting overload cut-out which may be substituted for that of Figure 1.

Figure 3 is a diagrammatic illustration of still another form of automatic resetting overload cut-out which may also be substituted for that shown in Figure 1. I

Referring now to Figure l, a mechanical refrigerating apparatus is generally designated at ID for maintaining a desired condition in a space H. The space H may be a walk-in box, a storage compartment, or any other space the condition of which is to be controlled. The mechanical refrigerating apparatus may comprise a compressor it operated by an electric motor l3. Refrigerant is compressed in the compressor 12 and passed through a high pressure line I I to a condenser I5 where it is condensed. The condensed refrigerant passes from the condenser I5 into a receiver 16 and from there through a liquid line l1 and an expansion valve |8 into an evaporator |9 located in the space H. The refrigerant is expanded in evaporator l9 and is withdrawn therefrom by the compressor l2 through a suction line 20. The expansion valve I8 is shown for purposes of illustration to be a thermostatic expansion valve having a capillary tube 2| connected to a bulb 22 containing a volatile fluid and located at the discharge side of the evaporator I9.

The compressor motor l3 may be controlled by a unitary control arrangement generally designated at 25 which may be of the type shown and described in the above referred to Judson and Kronmiller application. This unitary control arrangement may comprise a base or casing 26 in which is mounted a bellows 21 connected by a pipe 28 to the suction or low pressure line 29. The bellows 21 operates a lever 29 pivoted on a fulcrum member 38 against the action of a tension spring 3|. One end of the tension spring 3| is connected to the lever 29 and the other is connected to a nut 32 screw threadedly mounted on a screw 33. By rotating the screw 33 the tension of the spring 3| may be varied and hence the pressure setting of the bellows 21 may be adjusted at will. Lever 29 carries an insulating pad 34 upon which is mounted a contact 35 adapted to engage a contact member 36 suitably anchored to terminal 31 and a contact 38 adapted to engage a contact member 39 suitably secured to a terminal 48. The contact members 38 and 39 are adapted to be independently positioned by means of concentrically located cams 4| and 42. For purposes of illustration, it is assumed that the cams 4| and 42 are so adjusted that the contact 35 engages the. contact member 36 when the suction pressure rises to 15 lbs. and that the contact 38 engages the contact member 39 when the suction pressure rises to a higher value of say 35 lbs., this value corresponding to the defrosting temperature of the evaporator. Hence the contact 38 can engage the contact member 39 only after defrosting of the evaporator I9 has occurred.

Also mounted in the casing 26 is a bellows 45 connected by a pipe 46 to the high pressure line I4 01 the mechanical refrigerating apparatus. The bellows 45 operates a. lever 41 fulcrumed on a fulcrum member 48 against the action of a tension spring 49. One end of the tension spring 49 is connected to the lever 41 and the other end is connected to a nut 58 screw threadedly mounted on a screw 5|. By rotating the screw 5| the tension of the spring 49 is varied and hence the pressure setting of the bellows 45 may be adjusted at will. Lever 41 carries an abutment member 52 having an abutment surface 53 adapted to engage contact member 54 suitably mounted on a terminal 55. The contact member 54 engages and disengages a contact terminal 56. The abutment member 52 is also provided with an abutment surface 51 which is adapted to engage a. contact member 58 also secured at terminal 49. Contact member 58 is adapted to engage and disengage a contact post 59. For purposes of illustration, it is assumed that the parts are so arranged that the contact member 58 and the contact post 59 engage and disengage at a head pressure of substantially 130 lbs. and that the contact member 54 and the contact terminal 56 engage and disengage at a head pressure of substantially 180 lbs.

The unitary control arrangement 25 also includes a relay or starter generally designated at 6| which may comprise an operating coil 62 for operating a bridge member 63 with respect to contact terminals 64 and 65 and for operating a bridge member 66 with respect to contact posts 61 and 68. When the operating coil 62 is energized the bridge member 63 is moved into engagement with the contact terminals 64 and 65 and the bridge member 66 is moved into engagement with the contact posts 61 and 68. When the operating coil 62 is deenergized, the bridge members 63 and 66 are moved out of engagement with their respective contacts by means of springs, gravity, or other means, not shown. The unitary control arrangement 25 is also provided with a load terminal 69 and a control terminal 19 which is electrically connected to the contact post 61. In this instance the control terminal 18 is not utilized.

to the load terminal 69 respectively. The compressor circuit includes conductors 14 and 15 extending from the contact terminal 65 and load terminal 69 to the compressor motor terminals 16 and 11 respectively.

For purposes of maintaining substantially constant conditions within the space H, such as temperature conditions, a temperature controller responding to variations in space temperature is generally designated at 88. This temperature controller 88 may comprise a bellows 8| containing a volatile fluid for operating a lever 82 against the action of a tension spring 83. The lever 82 operates a mercury switch 84 having electrodes 85 and 86. The arrangement is such that when the temperature of the space H increases the bellows 8| expands to cause the mercury of the mercury switch 84 to bridge the electrodes 85 and 86, and upon a decrease in temperature the mercury is caused to separate from the electrodes 85 and 86. By suitably adjusting the tension in the spring 83 the temperature setting of the temperature responsive controller 89 may be adjusted at will. For purposes of iilustration, it is assumed that the switch 84 is closed when the space temperature rises to 40 and is opened when the space temperature decreases to 38.

The automatic resetting overload cut-out is shown to be located in the compressor motor l3 so as to be influenced by the temperature thereof. This overload cut-out means may comprise a terminal 88 upon which is mounted a bimetallic element 89 for opening and closing contacts 90. Upon the occurrence of an overload condition, the bimetallic element 89 is flexed to the left to separate the contacts 99 and when the overload condition has ceased to exist, the bimetallic element 89 moves to the right to re-close the contacts 99.

With the parts in the position shown in the drawing, the relay or starter 6| is deenerglzed and the compressor motor i3 is not operating. As a result, the suction pressure increases as does the space temperature and the head pressure decreases. When the space temperature rises to 40, the suction pressure rises to 35 lbs. and the head pressure decreases to lbs., a circuit is completed from the line wire 12 through contact terminal 66, contact member '4, terminal 55, terminal 88, bimetallic element 88 and contacts 98 of the overload cut-out, electrodes 86 and 86 of the temperature responsive controller 88, terminal 31, contact member 36, contacts 35 and 38, contact member 39, contact member 58, contact post 59, contact post 68, operating coil 62 and terminal 69 back to the other line Wire 13. Completion of this circuit causes energization of the operating coil to move the bridge member 63 into engagement with the contact terminals 64 and 65 and to move the bridge member 66 into engagement with the contact posts 67 and 68.

Movement of the bridge member 63 into engagement with the contact terminals iii and 65 completes a load circuit'for the compressor from the line wire it through contact terminal 56, contact terminal 6 t, bridge member contact terminal 65, wire i i, compressor motor terminals' i6 and El, wire it and terminal 69 back to the line it. Accordingly, when the operating coil 152 is energized the compressor motor i3 is placed in operation.

Upon movement of the bridge member into engagement with the contact posts till and as a result of energization of the operating coil 62, a maintaining circuit is completed from the line wire it through contact terminal contact member 5%, terminal terminal til, bimetallic element til and contacts dd of the overload cutout, electrodes t5 and iii? of the temperature responsive controller Elli, terminal 3i,'contact member contact contact post 6?, bridge member contact post operating coil and terminal 69 back to the other line wire l3. Completion of this circuit maintains the relay or starter iii energized and hence maintains the compressor motor in operation even though the suction pressure decreases below 35 lbs. and the high pressure increases above lfiil lbs.

l'i now either the space temperature decreases to 38, the suction pressure decreases to 15 lbs., the high pressure increases to ltd lbs., or an overload condition occurs in the compressor motor it, the relay or starter [ii is deenergizeol to stop operation of the compressor motor 03. The compressor motor then cannot be restarted until the above rererrecl to starting circuit is completed, which can occur only when the suction pressure rises to a defrosting value of 35 lbs. and the high pressure decreases to a predetermined low value of lbs. In other words, the compressor may not be restarted after a shut-down until the evaporator it has defrosted and until the high pressure has decreased to 130 lbs;

Assume that the temperature responsive controller 8b is calling for cooling and that the cornpressor motor 03 is operating. If an overload condition occurs in the compressor motor 63 the relay or starter iii is deenergized since the over load cut-out is included inv both the starting and maintaining circuits. When the overload condi= tion no longer occurs the contacts til of the overload cut out remake but the compressor can-= not again be placed in operation until the suction pressure has increased to the defrosting value of 35 lbs. and the high pressure hasdecreased to the predetermined low value of 13d lbs. As

a result, cycling of the compressor on the overload cut-out regardless of evaporator temperature cannot occur since the compressor motor it cannot be restarted until the evaporator it has defrosted. Accordingly, the building up of large amounts of frost on the evaporator iii is entirely prevented. Also, when the load condition no longer occurs the compressor motor it cannot be placed in operation until the head pressure has decreased to a predetermined low value of by the making and breaking of contacts 9t 01.

the overload cut-out cannot occur and hence the necessity of a snap action type overload cut-out 1 is not required. Accordingly, a simple overload cut-out of the automatic resetting type may be utilized without any danger of short cycling of the compressor motor iii.

Preferably the automatic resetting overload cut-out is included in the compressor motor cas ing so that it will be influenced by the temperature of the compressor motor is. Compressor motors are usually provided with air circulating means for cooling the same during operation thereof. When an overload condition occurs during operation so as to open the overload cut-= out and stop the compressor motor, the air circulating means also stops and the temperature of the compressor motor increases to insure that the cut=out remains open for a given time interval until the. compressor motor temperature decreases to a safe operating value. This further prevents short cycling oi the compressor motor by the overload cut-ont and also eliminates the necessity of snap action overload cut-outs.

Various types of automatic resetting overload cut-outs may be utilized; such as those shown in Figures l, 2 and 3. In Figure l the overload cut out responds solely to compressor motor temperature. in Figure 2 the overload cut-out is not only responsive to compressor motor temperatures but is also controlled by the current flow through the compressor motor. this connection, heater 592 may be connected between the motor terminal ill and the wire to additionally heat the thermostatic element 89 of the overload cut o-ut. With such an arrangement, the overload cut-out may be made more responsive to instantaneous overload conditions, the thermostatic element being additionally heated by the heater element In Figme 3 is shown another type of overload cut-out arrangement wherein the thermostatic element may additionallybe heated by inductance from the motor windings The motor windings usually extend outwardly from the core of the motor and by locating the thermostatic element adjacent these windings an induced current is caused to take place in the thermostatic element 89 which heats: the same in accordance with the value of the current passing through the motor windings. Accordingly, here as in the modification shown by Figure 2 the thermostatic element 89 of the overload cut-out is additionally heated in accordance with the current flow through the compressor motor.

From the above it is seen that I have provided a control system for a mechanical refrigcrating apparatus embodying an automatic resetting overload cut-out in combination with a control apparatus which preventsstarting of the compressor until defrosting has occurred and until the high or head pressure decreases to a minimum value whereby (1) upon the occurrence of an overload condition the compressor motor cannot be restarted until defrosting of the evaporator has occurred, whereby building up of frost on the evaporator with consequent decrease in efliciency of the refrigerating apparatus is'prevented, (2) upon the occurrence of an overload condition the compressor motor cannot be restarted until the high or head pressure decreases to a minimum whereby the starting torque is decreased to a minimum, which eliminates one of the main causes of overload shut-downs, and (3) rapid successions of starting and stopping of the compressor motor are entirely eliminated and the use of snap action overload cut-outs is not required.

Although for purposes of illustration one form of this invention has been disclosed, other forms thereof may become apparent to those skilled in the art upon reference to this disclosure and therefore this invention is to be limited only by the scope of the appended claims and prior art.

I claim as my invention:

1. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, comprising in combination, control means for starting and stopping the compressor motor, automatic resetting overload cut-out means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting of the compressor motor when the overload condition no longer exists, and means responsive to a condition resulting from the presence of frost on the evaporator for preventing starting of the compressor motor until defrosting has been substantially completed whereby cycling of the compressor motor by the automatic resetting overload cut-out means with consequent continued building up of frost on the evaporator is prevented.

2. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, comprising in combination, control means for starting and stopping the compressor motor, automatic resetting overload cut-out means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting of the compressor motor when the overload condition no longer exists, and means responsive to variations in suction pressure of the refrigerating apparatus for preventing starting of the com pressor motor until the suction pressure has risen to a value such that defrosting of the evaporator has occurred whereby cycling of the compressor motor by the automatic resetting overload cutout means with consequent continued building up of frost on the evaporator is prevented.

3. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, comprising in combination, control means for starting and stopping the compressor motor, automatic resetting overload cut-out means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting of the compressor motor when the overload condition no longer exists, and means responsive to variations in a condition which is a measure of head pressure on the high pressure side of the refrigerating apparatus for preventing starting of the compressor motor until the head pressure has decreased to a predetermined low value means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting of the compressor motor when the overload condition no longer exists, means responsive to a condition resulting from the presence of frost on the evaporator for preventing starting of the compressor motor until defrosting has been substantially completed whereby cycling of the compressor motor by the automatic resetting overload cut-out means with consequent continued building up of frost on the evaporator is prevented, and means responsive to variations in a condition which is a measure of head pressure on the high pressure side of the refrigerating apparatus for preventing starting of the compressor motor until the head pressure has decreased to a predetermined low value whereby the automatic resetting overload cut-out means cannot restart the compressor motor against a high head pressure.

5. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, comprising in combination, control means for starting and stopping the compressor motor, automatic resetting overload cut-out means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting of the compressor motor when the overload condition no longer exists, means responsive to variations in suction pressure of the refrigerating apparatus for preventing starting of the compressor motor until the suction pressure has risen to a value such that defrosting of the evaporator has occurred whereby cycling of the compressor motor by the automatic resetting overload cut-out means with consequent continued building up of frost on the evaporator is prevented, and means responsive to variations in head pressure on the high pressure side of the refrigerating apparatus for preventing starting of the compressor motor until the head pressure has decreased to a predetermined low value whereby the automatic rev setting overload cut-out means cannot restart the compressor motor against a high head pressure.

6. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, comprising in combination, control means for starting and stopping the compressor vmotor, automatic resetting overload cut-out whereby the automatic resetting overload cutout means cannot restart the, compressor motor until the starting torque has been reduced to or below a predetermined minimum.

7. A control system for a mechanical refrigerating apparatus having an evaporator, a-compressor and a motor for operating the compressor, comprising in combination, control means for starting and stopping the compressor motor, automatic resetting overload cut-out meanslocated in the compressor motor so as to be influenced by the temperature thereof for stopping the compressor motor when an overload condition of the compressor motor occurs and allowing restarting of the compressor motor when the temperature thereof decreases to a predetermined value, and means responsive to a condition resulting from the presence of frost on the evaporator for preventing starting of the compressor motor until defrosting has been substantially completed whereby cycling of the com-= pressor motor by the automatic resetting overload cut-out means with consequent continued building up of frost on the evaporator is prevented.

8. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, comprising in combination, control means for starting and stopping the compressor motor, automatic resettin overload cut-out means located in the compressor motor so as to be influenced by the temperature thereof for stopping the compressor motor when an overload condition. of the compressor motor occurs and allowing restarting or" the compressor motor when the temperature thereof decreases to a predetermined value, and means responsive to variations in a condition which is a measure of head pressure on the high pressure side of the refrigerating apparatus for preventing starting of the compressor motor until the head pressure has decreased to a predetermined low value whereby the automatic resetting overload cut out means cannot restart the compressor motor against a high head pressure.

9. A control system for a mechanical refrigerating apparatus having an evaporator, a com pressor and a motor for operating the cmnpres-= sor, comprising in combination, control means for starting and stopping the compressor motor, automatic resetting overload cut-out means located in the compressor motor so as to be influenced by the temperature thereof for stopping the compressor motor vhen an overload condition of the compressor motor occurs and allowin restarting of the compressor motor when the temperature thereof decreases to a predetermined value, and means responsive to variations in suction pressure and head pressure of the refrigerating apparatus for preventing starting of the compressor motor until the difierential be-= tween suction pressure and head pressure has decreased to at least a predetermined minimum whereby the automatic resetting overload cutout means cannot restart the compressor motor until the starting torque is reduced to or below a predetermined minimum.

10. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, control means including means responsive to changes in a psychrometric condition in a space for starting and stopping the compressor motor-to maintain said condition in the space within desired limits, automatic resetting overload cut-out means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting of the compressor motor when the overload condition no longer exists, and means responsive to a condition resulting from the presence of frost on the evaporator for preventing starting of the compressor motor until defrosting has been substantially completed whereby cycling of the compressor motor by the automatic resetting overload cut-out means with consequent continued building up of frost on the evaporator is prevented.

11. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, control means including means responsive to changes in a psychrometric condition in a space for starting and stopping the compressor motor to maintain said condition in the space within desired limits, automatic resetting overload cut-out means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting of the compressor motor when the overload condition no longer exists, and means responsive to variations in a condition which is a measure of head pressures on the high pressure side of the refrigerating apparatus for preventing starting of the compressor motor until the head pressure has decreased to a predetermined low value whereby the automatic resetting overload cut-=out means cannot restart the compressor motor against a high head pressure.

12: A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, control means including means responsive to changes in a psychrometric condition in a space for starting and stopping the compressor motor to maintain the condition in the space within, desired limits, automatic resetting overload cut-out means for stopping the compressor motor and operated by an overload condition of the compressor motor and which allows restarting or the compressor motor when the overload condition no longer exists, and means responsive to variations in suction pressure and head pressure of the refrigerating apparatus for preventing starting of the compressor motor until the differential between suction pressure and head pressure has decreased to at least a predetermined minimum whereby the automatic resetting overload cut-out means cannot restart the compressor motor until the starting torque is reduced to or below a predetermined minimum.

13. A. control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, control means including means responsive to changes in a psychrometric condition in a space for starting and stopping the compressor motor to maintain the condition in the space Within desired limits, automatic resetting overload cutout means located in the compressor motor so as to be influenced by the temperature thereof for stopping the compressor motor when an overload condition occurs and allowing restarting of the compressor motor when the temperature thereof decreases to a predetermined value, and means responsive to a condition resulting from the presence of frost on. the evaporator for preventing starting of the compressor motor until defrosting has been substantially completed whereby cycling of the compressor motor by the automatic resetting overload cut-out means with consequent continued building up of frost on the evaporator is prevented.

14. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, control means including means responsive to changes in a psychrometric condition in a space for starting and stopping the compressor motor to maintain the condition in the space within desired limits, automatic resetting overload cutout means located in the compressor motor so as to be influenced by the temperature thereof for stopping the compressor motor when an overload condition occurs and allowing restarting of the compressor motor when the temperature thereof decreases to a predetermined value, and means responsive to variations in a condition which is a measure of head pressures on the high pressure side of the refrigerating apparatus for preventing starting of the compressor motor until the head pressure has decreased to a predetermined low value whereby the automatic resetting overload cut-out means cannot restart the compressor motor against a high head pressure.

15. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and a motor for operating the compressor, control means including means responsive to changes in a psychrometric condition in a space for starting and stopping the compressor motor to maintain the condition in the space within desired limits, automatic resetting overload cutout means located in the compressor motor so as to be influenced by the temperature thereof for stopping the compressor motor when an overload condition occurs and allowing restarting of the compressor motor when the temperature thereof decreases to a predetermined value, and means responsive to variations in suction pressure and head pressure of the refrigerating apparatus for preventing starting of the compressor motor until the differential between suction pressure and head pressure has decreased to at least a predetermined minimum whereby the automatic resetting overload cut-out means cannot restart the compressor motor until the starting torque is reduced to at least a predetermined minimum. 16. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and an electric motor for operating the compressor, comprising in combination, a relay including an operating coil, a maintaining switch and a load switch for the compressor, the arrangement being such that when the operating coil is energized the maintaining switch and the load switch are closed, automatic resetting overload means including a first switching means and operable to open the same when an overload condition occurs in the compressor motor and to close the same when the overload condition no longer occurs, second switching means, means responsive to a condition resulting from the presence of frost on the evaporator for closing the second switching means when the evaporator has been substantially defrosted, a starting circuit for the operating coil of the relay including the first and second switching means for starting the compressor motor when both switching means are closed, and a maintaining circuit for the operating coil of the relay including the maintaining switch and the first switching means for maintaining the compressor motor in operation, the arrangement being such that upon the occurrence of an overload condition the compressor motor cannot be restarted until defrosting of the evaporator has been substantially completed.

17. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and an electric motor for operating the compressor, comprising in combination, a relay including an operating coil, a maintaining switch and a load switch for the compressor, the arrangement being such that when the operating coil is energized the maintaining switch and the load switch are closed, automatic resetting overload means including a first switching means and operable to open the same when an overload condition occurs in the compressor motor and to close the same when the overload condition no longer occurs, second switching means, means responsive to a condition which is a measure of head pressure of the refrigerating apparatus for closing the second switching means when the head pressure decreases to a predetermined low value, a starting circuit for the operating coil of the relay including the first and second switching means for starting the compressor motor when bothswitching means are closed, and a maintaining circuit for the operating coil of the relay including the maintaining switch and the first switching means for maintaining the compressor motor in operation, the arrangement being such that upon the occurrence of an overload condition the compressor motor cannot be restarted until the head pressure decreases to the predetermined low value whereby starting of the compressor against high head pressure is prevented.

18. A control system for a mechanical refrigerating apparatus having an evaporator, a compressor and an electric motor for operating the compressor, comprising in combination, a relay including an operating coil, a maintaining switch and a load switch for the compressor, the arrangement being such that when the operating coil is energized the maintaining switch and the load switch are closed, automatic resetting overload means including a first switching means and operable to open the same when an overload condition occurs in the compressor motor and to close the same when the overload condition no longer occurs, second switching means, means responsive to variations in suction pressure for closing the second switching means when the suction pressure increases to a predetermined high value, third switching means, means responsive to variations in head pressure for closing the third switching means when the head pressure decreases to a predetermined low value, a starting circuit for the operating coil of the relay including the first, second and third switching means for starting the compressor motor when these switching means are closed, and a maintaining circuit for the operating coil of the relay including the maintaining switch and the first switching means for maintaining the compressor motor in operation, the arrangement being such that upon the occurrence of an overload condition the compressor motor cannot be restarted until the differential in pressure between the suction pressure and the head pressure decreases to or below a predetermined minimum.

ALWIN B. NEWTON. 

